Optical components have at least two surfaces, a front surface and a rear surface. The front surface is an object facing surface and the rear surface is an image facing surface. During fabrication of optical components, such as prisms, aspheric (Schmidt) plates, lens components (such as single lens components or cemented lens components), it is desirable to control and thus to measure the relative position of the front and rear surfaces with respect to one another.
A lens component may have spherical or aspherical surfaces. A lens surface is usually defined by the location of its vertex, the direction of its optical axis (which is normal to the surface at the vertex), and an aspheric equation including aspheric coefficients describing that surface. In a spherical surface the aspheric coefficients are zero. A lens component with two aspheric surfaces is called a biaspheric lens component. In a lens component, the relative angular separation between the two optical axes (of its respective surfaces) is referred to as wedge (see FIG. 1). The shortest distance between the two optical axes is called decenter. This distance is usually measured along the shortest line that connects one optical axis to the other optical axis, and that line originates at one of the surface vertices.
Measuring devices for measuring individual surface shapes are known. Both surfaces of a lens component need to be measured with such a device in order to determine the relative position of each surface with respect to the other surface. Since the surfaces are on opposite sides of a lens component it is difficult to measure the two surfaces without moving the lens component or the measuring device. Usually, an optical technician utilizes an optical or mechanical device that can measure characteristics of only one surface at a time. To measure the surfaces of the lens component relative to each other, the technician may keep the lens component stationary and move the measuring device around the lens component, measuring one surface at a time. Because the measuring device is much bigger and heavier than the lens component, this approach is inconvenient.
Mechanical surface measuring devices such as profilometers are known. Previously, most profilometers have performed measurements in two dimensions. These profilometers do not provide the high degree of accuracy required for measurements of surface profiles of precision optical components.
Recently, more accurate profilometers that measure in three dimensions have become available. Such profilometers are commercially available from Panasonic Factory Automation, Chicago, Ill. These profilometers are designed to measure aspherical or spherical surface profiles over a workspace of several inches. They use a single vertically oriented probe that travels in the vertical direction (up and down) as well as in a horizontal plane. These profilometers measure a surface profile on one surface of a lens component at a time and determine the orientation of an optical axis of that surface. However, in order to measure the orientation of two surfaces with respect to one another, the lens component must be turned over so that its second surface can also be measured. This movement of the lens component introduces an uncertainty in measuring the relative positions of the front surface of the lens component with respect to its rear surface and produces inaccurate results. The same problem occurs if one uses an interferometer or another measuring device, instead of a profilometer.